E≈S ournal of the European Ceramic Society 20(2000)589-597 Effect of coating deposition temperature on monazite coated fiber R.S. Haya, E Boakye, M.D. Petry lir Force Research Laboratory, Materials Directorate, Wright Patterson Air Force Base, OH 45433, USA 4401 Davi lia Road, Dayton, OH 45432 USA Accepted 13 August 1999 Abstract Monazite (apoA) was continuously coated on 3M Nextel 720 fiber tows with an ethanolic precursor using hexadecane for immiscible liquid displacement Coating deposition temperatures were varied from 900 to 1300.C. Fibers coated at 900C were heat-treated up to 100 h at 1200C. Coated fibers were characterized by analytical TEM, and tensile strengths were measured by structure evolution was complex and may have involved recrystallization of large defective grains into smaller grains and ther subsequent growth of these grains, along with coarsening of porosity. After 100 h at 1200 C there was significant roughening of the coating-fiber interface, with facetting of alumina grains in the fiber and some lanthanum segregation to these facetted boundaries Spheroidization of thin coatings was also observed. Tensile strength of coated fiber decreased with increasing deposition tempera- ture and with time at temperature after deposition. Possible reasons for the strength decrease are discussed lished by elsevier Science ltd Keywords: Aluminosilicate fibers; Coating: Grain growth; Interfaces; LaPO4: Mechanical properties 1. ntroduction itu between the fiber and matrix during processing. Sol and solution precursors have been used to coat fiber Improvement of intermediate and high temperature tows with monazite, as has continuous chemical vapor properties of ceramic matrix composites (CMCs) deposition(CVD). 10,15,16 Limited experiments demon- requires an oxidation resistant alternative to carbon or strate crack deflection and fiber pullout in tensile tests of boron nitride fiber-matrix interfaces. -6 One possible composites with CVD monazite coated fiber tows. 10 For alternative is monazite(LaPO4). Monazite is refractory liquid-phase precursors, crack deflection and debonding (mp 2072C)and thermochemically stable with many have only been demonstrated on composites with low other common refractory oxides such as alumina and volume fractions of dip coated, large diameter(100 um) mullite. Crack deflection and fiber pushout experi- single crystal alumina(Saphikon )monofilaments. The ments,as well as some limited mechanical tests on thickness uniformity of sol and solution derived coat composites suggest that monazite bonds weakly with ings on filaments in fiber tows is often poor, although other oxides 8-10 Some similar results were found for stoichiometry is easily controlled. 6 In contrast, limited xenotime(YPO4) and scheelite(CaWO4). 2 Monazite data suggest that CVD coatings have good thickness containing ceramics were demonstrated to be machine- uniformity, but are sometimes off stoichiometry. 15, 16 able, 3 which implies significant plasticity from some Filament tensile strength is often degraded during combination of cleavage microcracking, twinning, or fiber coating, 6. 17-19 or during exposure to various envir dislocation glide. 4 onments.20), 2I Strength increases after coating or envir Use of monazite as a fiber-matrix interface in MCs onmental exposure are also known 22.23 Large requires that it either be coated on fibers or formed in differences in tensile strength have been observed between fibers coated with different monazite pre- 4 Corresponding author Tel: 1-937 255 9825: fax: 1 937 6564296. strength dependence on coating temperature were also E- mail address: hayrs(@ ml wl wpafb af. mil(RS. Hay) found. Excessive degradation in fiber tensile strength 0955-2219/00/S- see front matter. Published by Elsevier Science Ltd PII:S0955-2219(99)00257-5
Eect of coating deposition temperature on monazite coated ®ber R.S. Haya,*, E. Boakyeb, M.D. Petryb a Air Force Research Laboratory, Materials Directorate, Wright Patterson Air Force Base, OH 45433, USA bUES Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432, USA Accepted 13 August 1999 Abstract Monazite (LaPO4) was continuously coated on 3M NextelTM 720 ®ber tows with an ethanolic precursor using hexadecane for immiscible liquid displacement. Coating deposition temperatures were varied from 900 to 1300C. Fibers coated at 900C were heat-treated up to 100 h at 1200C. Coated ®bers were characterized by analytical TEM, and tensile strengths were measured by single ®lament tensile tests. The monazite precursor was characterized by X-ray, DTA/TGA, and mass spectrometry. Microstructure evolution was complex and may have involved recrystallization of large defective grains into smaller grains and then subsequent growth of these grains, along with coarsening of porosity. After 100 h at 1200C there was signi®cant roughening of the coating±®ber interface, with facetting of alumina grains in the ®ber and some lanthanum segregation to these facetted boundaries. Spheroidization of thin coatings was also observed. Tensile strength of coated ®ber decreased with increasing deposition temperature and with time at temperature after deposition. Possible reasons for the strength decrease are discussed. Published by Elsevier Science Ltd. Keywords: Aluminosilicate ®bers; Coating; Grain growth; Interfaces; LaPO4; Mechanical properties 1. Introduction Improvement of intermediate and high temperature properties of ceramic matrix composites (CMCs) requires an oxidation resistant alternative to carbon or boron nitride ®ber-matrix interfaces.1±6 One possible alternative is monazite (LaPO4). Monazite is refractory (mp 2072C) and thermochemically stable with many other common refractory oxides such as alumina and mullite.7±9 Crack de¯ection and ®ber pushout experiments, as well as some limited mechanical tests on composites suggest that monazite bonds weakly with other oxides.8±10 Some similar results were found for xenotime (YPO4) 11 and scheelite (CaWO4).12 Monazite containing ceramics were demonstrated to be machineable,13 which implies signi®cant plasticity from some combination of cleavage microcracking, twinning, or dislocation glide.14 Use of monazite as a ®ber-matrix interface in CMCs requires that it either be coated on ®bers or formed in situ between the ®ber and matrix during processing. Sol and solution precursors have been used to coat ®ber tows with monazite,6 as has continuous chemical vapor deposition (CVD).10,15,16 Limited experiments demonstrate crack de¯ection and ®ber pullout in tensile tests of composites with CVD monazite coated ®ber tows.10 For liquid-phase precursors, crack de¯ection and debonding have only been demonstrated on composites with low volume fractions of dip coated, large diameter (100 mm) single crystal alumina (Saphikon1) mono®laments.8 The thickness uniformity of sol and solution derived coatings on ®laments in ®ber tows is often poor, although stoichiometry is easily controlled.6 In contrast, limited data suggest that CVD coatings have good thickness uniformity, but are sometimes o stoichiometry.15,16 Filament tensile strength is often degraded during ®ber coating,6,17±19 or during exposure to various environments.20,21 Strength increases after coating or environmental exposure are also known.22,23 Large dierences in tensile strength have been observed between ®bers coated with dierent monazite precursors.6 Preliminary indications of a strong tensile strength dependence on coating temperature were also found. Excessive degradation in ®ber tensile strength 0955-2219/00/$ - see front matter. Published by Elsevier Science Ltd. PII: S0955-2219(99)00257-5 Journal of the European Ceramic Society 20 (2000) 589±597 * Corresponding author Tel.:1-937 255 9825; fax: 1 937 656 4296. E-mail address: hayrs@ml.wl.wpafb.af.mil (R.S. Hay)
al / Journal of the European Ceramic Society 20(2000)589-59 may cause an otherwise functional coating to appear tem thin foils were made of coated fiber cross-sec- non-functional. Knowledge of the conditions under tions as described elsewhere 35.36 Thin foils were which degradation occurs is therefore important for observed in either a JEOL 2000 FX operating at 200 interface evaluation, besides the obvious importance to kv, or in a Phillips CM 200 FEG operating at 200 k composite strength and failure mechanisms. 24-26 Isolation Energy dispersive spectroscopy(EDS)measurements of the causes of strength degradation may lead to were done in the Phillips CM 200 FEG with a 5 nm spot improved coating methods that minimize this degradation. size and a windowless detector This work describes characteristics of 3M Nextel 720 Coated filament tensile strengths and Weibull modulii fiber coated with monazite between 900 and 1300C. were measured using a 2.54 cm gauge length with 75 Characteristics of the 900oC coated fiber after heat- tests. For controls, tensile strengths were also measured treatment for up to 100 h at 1200oC are also described. for filaments that had been heat-treated without coat Changes in composition and microstructure of the ing, and for filaments that had been passed through the coating and fiber were observed by TEM. Precursor and fiber coater under conditions that mimicked a coating coating evolution were monitored by X-ray, TEM, and run, but without coating deposition. Further details of DTA/TGA. Precursor gas evolution was observed by tensile strength measurement are presented elsewhere. 3 DTA/TGA and mass spectrometry. Strength of coated and uncoated fibers were measured by single filament tensile tests. Changes in coating characteristics with 3. Results temperature were compared with changes in the coated filament tensile strength. The results are analyzed and 3. 1. X-ray, DTA/TGA, and mass spectrometry possible degradation mechanisms are discussed Precursor heat-treated for l h at 1200 C has monazite y peaks and small La3PO7 peaks, indicating a slight phosphate deficiency from the monazite stoichiometry (ig. 1). A weak exotherm at about 450C and a weak Lanthanum nitrate and phosphorous pentoxide were endotherm at about 950C were the only significant dissolved in dry ethanol with the appropriate stoichio- DTA features(Fig. 2). About 4.5% of the precursor metry to form 50 g/l of monazite. 6 These solutions had a mass was lost between 100 and 550 C. Above 550C density of 0.84 g/cm and a viscosity of 1. 39 mPa- S, as mass loss was less rapid, but an additional 1.5%was measured in a Brookfield programmable rheometer still lost between 550 and 1500C. This 1.5% loss cor- ( model DV-IID) at a shear rate of 1/300. The precursor responds to a volume of gas that is at least 10 to 50 was characterized after a I h heat- treatment at 1200c times larger than the volume of monazite it evolved by X-ray diffraction in a Rigaku Rotaflex Dif- from. A slight increase in mass loss at around 950C fractometer. Differential thermal analysis (DTA)and correlates with the temperature of the weak DTA thermogravimetric analysis (TGA) were done in a endotherm. By mass spectrometry, only HO, Nor Netzsch STA-409 at 10C/min up to 1500 C after the CO, and N2o or Co? were observed to evolve in sig precursor was dried for I h at 140 C. Mass spectro- nificant quantities above 600 C(Fig. 3). Lack of amu metry of gases evolved from the precursor was done in a resolution precluded distinguishing N2 from Co(amu Balzers QMs 420 at 5C/min up to 1050C. The mea- 28)or N20 from CO2(amu 44). Methane(CH4) and surement was done in argon so measurement overlap CH3 evolution at 500C roughly corresponds to the with atmospheric gases could be eliminated. A more detailed description of the mass spectrometry equip- ment is given elsewhere. 27 3M Nextel 720 alumina-mullite fibers 28-30 were con- tinuously coated with the monazite precursor in a ver- tical coater using hexadecane for immiscible liquid displacement. A coating speed of 1. 4 cm/s in an air atmosphere was used for all experiments. The furnace hot zone was about 8 cm in length, and total furnace length was 30 cm. The fiber coating apparatus and pro- cedures are described in more detail elsewhere 6.31-34 The fibers were desized in air at 1000C and 2.8 cm/s before coating. Coating runs were done at 900, 1000, 1 100, 1200, and 1300 C. Some fibers coated at 900C Fig. 1. X-ray diffraction pattern of monazite from precursor heat were then heat-treated at 1200%C for 0.2. 2. 10. or 100 h treated for I h at 1200C. All peaks correspond to monazite except in a furnace with MoSi, heating elements those highlighted in gray, which correspond to La
may cause an otherwise functional coating to appear non-functional. Knowledge of the conditions under which degradation occurs is therefore important for interface evaluation, besides the obvious importance to composite strength and failure mechanisms.24±26 Isolation of the causes of strength degradation may lead to improved coating methods that minimize this degradation. This work describes characteristics of 3M Nextel 720 ®ber coated with monazite between 900 and 1300C. Characteristics of the 900C coated ®ber after heattreatment for up to 100 h at 1200C are also described. Changes in composition and microstructure of the coating and ®ber were observed by TEM. Precursor and coating evolution were monitored by X-ray, TEM, and DTA/TGA. Precursor gas evolution was observed by DTA/TGA and mass spectrometry. Strength of coated and uncoated ®bers were measured by single ®lament tensile tests. Changes in coating characteristics with temperature were compared with changes in the coated ®lament tensile strength. The results are analyzed and possible degradation mechanisms are discussed. 2. Experiments Lanthanum nitrate and phosphorous pentoxide were dissolved in dry ethanol with the appropriate stoichiometry to form 50 g/l of monazite.6 These solutions had a density of 0.84 g/cm3 and a viscosity of 1.39 mPa.s, as measured in a Brook®eld programmable rheometer (model DV-III) at a shear rate of 1/300. The precursor was characterized after a 1 h heat-treatment at 1200C by X-ray diraction in a Rigaku Rota¯ex Diffractometer. Dierential thermal analysis (DTA) and thermogravimetric analysis (TGA) were done in a Netzsch STA-409 at 10C/min up to 1500C after the precursor was dried for 1 h at 140C. Mass spectrometry of gases evolved from the precursor was done in a Balzers QMS 420 at 5C/min up to 1050C. The measurement was done in argon so measurement overlap with atmospheric gases could be eliminated. A more detailed description of the mass spectrometry equipment is given elsewhere.27 3M Nextel 720 alumina±mullite ®bers28±30 were continuously coated with the monazite precursor in a vertical coater using hexadecane for immiscible liquid displacement. A coating speed of 1.4 cm/s in an air atmosphere was used for all experiments. The furnace hot zone was about 8 cm in length, and total furnace length was 30 cm. The ®ber coating apparatus and procedures are described in more detail elsewhere.6,31±34 The ®bers were desized in air at 1000C and 2.8 cm/s before coating. Coating runs were done at 900, 1000, 1100, 1200, and 1300C. Some ®bers coated at 900C were then heat-treated at 1200C for 0.2, 2, 10, or 100 h in a furnace with MoSi2 heating elements. TEM thin foils were made of coated ®ber cross-sections as described elsewhere.35,36 Thin foils were observed in either a JEOL 2000 FX operating at 200 kV, or in a Phillips CM 200 FEG operating at 200 kV. Energy dispersive spectroscopy (EDS) measurements were done in the Phillips CM 200 FEG with a 5 nm spot size and a windowless detector. Coated ®lament tensile strengths and Weibull modulii were measured using a 2.54 cm gauge length with 75 tests. For controls, tensile strengths were also measured for ®laments that had been heat-treated without coating, and for ®laments that had been passed through the ®ber coater under conditions that mimicked a coating run, but without coating deposition. Further details of tensile strength measurement are presented elsewhere.37 3. Results 3.1. X-ray, DTA/TGA, and mass spectrometry Precursor heat-treated for 1 h at 1200C has monazite X-ray peaks and small La3PO7 peaks, indicating a slight phosphate de®ciency from the monazite stoichiometry (Fig. 1). A weak exotherm at about 450C and a weak endotherm at about 950C were the only signi®cant DTA features (Fig. 2). About 4.5% of the precursor mass was lost between 100 and 550C. Above 550C mass loss was less rapid, but an additional 1.5% was still lost between 550 and 1500C. This 1.5% loss corresponds to a volume of gas that is at least 10 to 50 times larger than the volume of monazite it evolved from. A slight increase in mass loss at around 950C correlates with the temperature of the weak DTA endotherm. By mass spectrometry, only H2O, N2 or CO, and N2O or CO2 were observed to evolve in signi®cant quantities above 600C (Fig. 3). Lack of amu resolution precluded distinguishing N2 from CO (amu 28) or N2O from CO2 (amu 44). Methane (CH4) and CH3 evolution at 500C roughly corresponds to the Fig. 1. X-ray diraction pattern of monazite from precursor heattreated for 1 h at 1200C. All peaks correspond to monazite except those highlighted in gray, which correspond to La3PO7. 590 R.S. Hay et al. / Journal of the European Ceramic Society 20 (2000) 589±597
R.S. Hay et al. Journal of the European Ceramic Society 20(2000)589-597 DTA TGA Fig. 2. DTA/TGA of monazite precursor Scale for DTA trace is not 100nm ictures of typical coating anomalies: (a) Debonded n, poorly crystallize coating;(c) Broken filament to filament bridge. 3.3 The most obvious change in the coatings as deposi tion temperature increased from 900 to 1300C was a 100 300500700 decrease in pore volume fraction and an increase in pore size from 8 to 25 nm in diameter(Fig. 5). There were Temperature(C) other more subtle differences in coating microstructure. For coatings deposited at 900C, SAD patterns from Fig 3. Mass spectrometry of gas evolution from monazite precurso the coatings were roughly consistent with monazite, but diffraction spots were diffuse and streaked, and a fine <50 nm)grain size was inferred from the patterns Diffraction contrast did not change abruptly across DTA exotherm at about 450 C. The mass spectrometry grains as the sample was tilted; instead, diffuse diffraction specimen was black after the measurement, suggesting contrast swept irregularly across the coating, and iden- incomplete oxidation of carbonaceous species in the tification of specific grains was difficult. Evidently the argon environment. monazite coating grains were highly defective, but the defects could not be identified. For coatings deposited 3.2. Coatings-general at 1300C, diffraction patterns were sharp, with individual grains easily identifiable. Defects other than intra Coating thickness and morphology was similar to that granular porosity were not observed. The coating grain observed previously for this precursor. Coating cover- size was often much greater than the coating thickness age was continuous. Coating bridges between filaments thin poorly crystallized coatings, and debonded or 3.3.1. 1200C heat-treatments buckled"coatings were occasionally observed(Fig. 4) Coatings deposited at 900C and then heat-treated at Significant place to place variations in pore volume 1200oC had somewhat different microstructures. Two fraction and size were also observed. Coating thicknes hours at 1200.C changed the defective, porous, and averaged about 80 nm, with variations between 30 and poorly crystallized monazite grains(Fig. 5)to small( 120 common. Although a small amount of La PO, was nm diameter), well crystallized monazite grains with a observed by X-ray, no evidence of any phase other than small amount of porosity(Fig. 6). One hundred hours monazite was found in fiber coatings by tEM at 1200C caused grain growth to 100 nm diameter
DTA exotherm at about 450C. The mass spectrometry specimen was black after the measurement, suggesting incomplete oxidation of carbonaceous species in the argon environment. 3.2. Coatings Ð general Coating thickness and morphology was similar to that observed previously for this precursor.6 Coating coverage was continuous. Coating bridges between ®laments, thin poorly crystallized coatings, and debonded or ``buckled'' coatings were occasionally observed (Fig. 4). Signi®cant place to place variations in pore volume fraction and size were also observed. Coating thickness averaged about 80 nm, with variations between 30 and 120 common. Although a small amount of La3PO7 was observed by X-ray, no evidence of any phase other than monazite was found in ®ber coatings by TEM. 3.3. Coating temperature The most obvious change in the coatings as deposition temperature increased from 900 to 1300C was a decrease in pore volume fraction and an increase in pore size from 8 to 25 nm in diameter (Fig. 5). There were other more subtle dierences in coating microstructure. For coatings deposited at 900C, SAD patterns from the coatings were roughly consistent with monazite, but diraction spots were diuse and streaked, and a ®ne (<50 nm) grain size was inferred from the patterns. Diraction contrast did not change abruptly across grains as the sample was tilted; instead, diuse diraction contrast swept irregularly across the coating, and identi®cation of speci®c grains was dicult. Evidently the monazite coating grains were highly defective, but the defects could not be identi®ed. For coatings deposited at 1300C, diraction patterns were sharp, with individual grains easily identi®able. Defects other than intragranular porosity were not observed. The coating grain size was often much greater than the coating thickness. 3.3.1. 1200C heat-treatments Coatings deposited at 900C and then heat-treated at 1200C had somewhat dierent microstructures. Two hours at 1200C changed the defective, porous, and poorly crystallized monazite grains (Fig. 5) to small (40 nm diameter), well crystallized monazite grains with a small amount of porosity (Fig. 6). One hundred hours at 1200C caused grain growth to 100 nm diameter Fig. 2. DTA/TGA of monazite precursor. Scale for DTA trace is not given. Fig. 3. Mass spectrometry of gas evolution from monazite precursor. Fig. 4. TEM pictures of typical coating anomalies: (a) Debonded coating; (b) Thin, poorly crystallize coating; (c) Broken ®lament to ®lament coating bridge. R.S. Hay et al. / Journal of the European Ceramic Society 20 (2000) 589±597 591
.S. Hay et al. / Journal of the European Ceramic Society 20(2000)589-59 with even less porosity. The most obvious features after 3.4. Tensile strength 100 h at 1200oC were spheroidization of coatings less than 50 nm thick(Fig. 7), roughening of the monazite Coated filament tensile strength decreased from 2 coating-fiber interface( Figs. 6-8), and facetting of alu- GPa28 with both increased deposition temperature and mina grains in the fiber and near the coating along basal with time at 1200 C after deposition at 900 C ( Table 1 planes(Figs. 6-8). Lanthanum segregation at facetted Fig. 10). Filaments coated at 1300C had average tensile alumina-mullite interfaces near the coating was mea- strengths of 1. 21 GPa. Filaments heat-treated for 100 h sured by analytical TEM(Fig 8). Phosphorous segre- at 1200 C after coating at 900oC had only 0.83 GPa gation was not found and lanthanum was not found in average tensile strength. The actual average may have the interior of alumina grains. Wetting thin films were been slightly lower, because some filaments were too not observed along monazite-alumina, monazite-mul- weak to mount in grips for tensile testing, and their lite, or alumina-mullite interphase boundaries, or along contribution to the average would certainly have made the corresponding triple junctions(Fig. 9) it lower. Filaments coated at 1200C were slightly stronger than those coated at 1100 C, as were filaments 900℃C heat-treated for 2 h at 1200oC compared to 12 min at 1200.C. These observations were anomalous to the overall trend; it is possible that they are related to tran- sient flaw healing, but more data is required to sub stantiate this Control experiments suggest that much of the strength drop may be unrelated to the presence of a coating. Filaments heat-treated for 100 h at 1200C without a coating had only 1.04 GPa tensile strength (Table 1, Fig. 10). Similarly, filaments passed through the coating furnace at 1300C and 1200oC had only 1.5 GPa tensile strength, although filaments passed through water, water/HNO3, ethanol, and water/ octanol were stronger (Table I, Fig. 10). The strength drop between fibers coated at 1000 and 1100C was steeper than others, and may correlate with the slight 1300℃C Fig. 5. TEM pictures of coating deposited at(a)900oC,(b)I (c)1100 C, (d)1300C Simulated and observed diffraction patt 50 nm 900C coating and observed diffraction pattern from a 1300C are to the rig 200nm 2000mm Fig. 6. TEM pictures of coat posited at 900C and later heat- treated at 1200C for 2 h or 10 Fig. 7. TEM pictures of spheroidized coatings
with even less porosity. The most obvious features after 100 h at 1200C were spheroidization of coatings less than 50 nm thick (Fig. 7), roughening of the monazite coating±®ber interface (Figs. 6±8), and facetting of alumina grains in the ®ber and near the coating along basal planes (Figs. 6±8). Lanthanum segregation at facetted alumina±mullite interfaces near the coating was measured by analytical TEM (Fig. 8). Phosphorous segregation was not found, and lanthanum was not found in the interior of alumina grains. Wetting thin ®lms were not observed along monazite±alumina, monazite±mullite, or alumina±mullite interphase boundaries, or along the corresponding triple junctions (Fig. 9). 3.4. Tensile strength Coated ®lament tensile strength decreased from 2 GPa28 with both increased deposition temperature and with time at 1200C after deposition at 900C (Table 1, Fig. 10). Filaments coated at 1300C had average tensile strengths of 1.21 GPa. Filaments heat-treated for 100 h at 1200C after coating at 900C had only 0.83 GPa average tensile strength. The actual average may have been slightly lower, because some ®laments were too weak to mount in grips for tensile testing, and their contribution to the average would certainly have made it lower. Filaments coated at 1200C were slightly stronger than those coated at 1100C, as were ®laments heat-treated for 2 h at 1200C compared to 12 min at 1200C. These observations were anomalous to the overall trend; it is possible that they are related to transient ¯aw healing, but more data is required to substantiate this. Control experiments suggest that much of the strength drop may be unrelated to the presence of a coating. Filaments heat-treated for 100 h at 1200C without a coating had only 1.04 GPa tensile strength (Table 1, Fig. 10). Similarly, ®laments passed through the coating furnace at 1300C and 1200C had only 1.5 GPa tensile strength, although ®laments passed through water, water/HNO3, ethanol, and water/1- octanol were stronger (Table 1, Fig. 10). The strength drop between ®bers coated at 1000 and 1100C was steeper than others, and may correlate with the slight Fig. 5. TEM pictures of coating deposited at (a) 900C, (b) 1000C, (c) 1100C, (d) 1300C. Simulated and observed diraction pattern of 900C coating and observed diraction pattern from a 1300C coating are to the right. Fig. 6. TEM pictures of coating deposited at 900C and later heattreated at 1200C for 2 h or 100 h. Fig. 7. TEM pictures of spheroidized coatings. 592 R.S. Hay et al. / Journal of the European Ceramic Society 20 (2000) 589±597
.S. Hay et al. /Journal of the European Ceramic Society 20 (2000)589-597 2 nm t山Hd WAlNi w Mus Kev(x100) Fig 8. TEM picture of an area around a monazite coating -fiber interface analyzed by TEM. The EDS spectra of spots 1, 2, and 3 on the image are shown to the right. Spot 3 at an alumina-mullite interphase boundary shows trace lanthanum. increase in coating mass loss observed at about 950.C. Although the temperature interval does not exactly match, the heating rate was higher during coating than during the DTA/TGA measurements, so the corre- ponding mass loss during coating should be at a higher temperature. However, there are alternative explana tions for this strength drop that will be discussed in the next section Coating fibers with monazite at 900 C using this par- ticular precursor preserves most of the original tensile strength. However, this would not eliminate tensile strength degradation if composites with these coated fibers were processed or used at or above 1200.C, as shown by tensile strengths of 900C coated fiber heat treated at 1200C(Fig. 10). However, control experiments on uncoated fiber heat -treated at 1200 c have tensile strengths nearly as low as coated fibers. Beneath 1300C grain growth is insignificant in Nextel 720.38 The sensi Fig. 9. High resolution TEM micrograph of a monazite-alumina- tivity of tensile strength to heat-treatment or coating ng mullite triple junction at a coating-fiber interface. No evidence of a atmosphere (Table 1)suggests that degradation is most wetting phase at the interface or triple junction was found
increase in coating mass loss observed at about 950C. Although the temperature interval does not exactly match, the heating rate was higher during coating than during the DTA/TGA measurements, so the corresponding mass loss during coating should be at a higher temperature. However, there are alternative explanations for this strength drop that will be discussed in the next section. 4. Discussion Coating ®bers with monazite at 900C using this particular precursor preserves most of the original tensile strength. However, this would not eliminate tensile strength degradation if composites with these coated ®bers were processed or used at or above 1200C, as shown by tensile strengths of 900C coated ®ber heattreated at 1200C (Fig. 10). However, control experiments on uncoated ®ber heat-treated at 1200C have tensile strengths nearly as low as coated ®bers. Beneath 1300C grain growth is insigni®cant in Nextel 720.38 The sensitivity of tensile strength to heat-treatment or coating atmosphere (Table 1) suggests that degradation is most Fig. 8. TEM picture of an area around a monazite coating Ð ®ber interface analyzed by TEM. The EDS spectra of spots 1, 2, and 3 on the image are shown to the right. Spot 3 at an alumina±mullite interphase boundary shows trace lanthanum. Fig. 9. High resolution TEM micrograph of a monazite±alumina± mullite triple junction at a coating±®ber interface. No evidence of a wetting phase at the interface or triple junction was found. R.S. Hay et al. / Journal of the European Ceramic Society 20 (2000) 589±597 593